The proposed study will investigate the metabolism of polyunsaturated fatty acids in ciliated protozoa; the relationship of cholesterol and other sterols to polyunsaturated fatty acid biosynthesis in these organisms and the function of polyunsaturated fatty acids in ciliate membranes. The molecular specifications required for a lipid supplement to support growth and normal cell surface morphology in Tetrahymena pyriformis unable to synthesize its own polyenoic fatty acids at 40.1 degree centigrade will be determined. Membraneous organelles will be isolated from normal and abnormal ciliates grown on different lipid supplements at elevated temperatures and the lipid and fatty acid composition will be analyzed employing conventional techniques of column, thin-layer, and gas-liquid chromatography. The morphological and physiological consequences of experimental alterations of the lipid composition of isolated membraneous organelles will be determined. A previously observed relationship between polyunsaturated fatty acids and cholesterol will be investigated at the enzyme level in a sterol-requiring ciliate, Tetrahymena setifera.